Diagnosing a low power/low MPG issue

thechoochlyman

Vendor , w/Business number
Joined
May 7, 2015
Location
Campbellsville, Kentucky
TDI
1997 B4 Sedan
This is my friend's car that I've been helping him along with - 2003 Jetta wagon auto. He said that his MPG dropped significantly in the last few months starting after he changed the fuel filter (going from about 45 to 35). Coincidence or not, I had him order a new filter from ID Parts, and he's going to install it soon. Drained the old one with no luck. According to VCDS the only fault code he has is for a glow plug. I also lowered the IQ value because it was sitting around 5.3.

I think his cat is stopped up. Power goes to crap once it's over a certain RPM, but has plenty of low end. The car has ~240k miles, and the exhaust looks like it has been cobbled up somewhat before. He doesn't have a lot of money to spend, but I wanted to see what else I could check before he spends money getting the exhaust worked on.

He did go to an exhaust shop to have them cut off the cat and stick in a test pipe to see if it improved things, but all they did was check it was an infrared thermometer and tell him it was fine because the cat was no hotter than the rest of the piping. I think that's a very unreliable way to tell if it's clogged, personally.

Sorry if this question has been asked at length... I didn't find a sticky or any satisfactory answers with a search.
 

flee

Veteran Member
Joined
Sep 19, 2011
Location
Chatsworth, CA
TDI
2002 Jetta GLS wagon
If that 45 mpg was correct, that is at the highest possible range for a 01M.
My milage has been only 36-38 mpg for all the years of ownership.
So, maybe chasing something as simple as a dragging brake caliper, bad alignment,
low tire pressure, etc.

Checking the cat is a pain but pretty low tech once you get the car up off the ground.
 

Tdijarhead

Top Post Dawg
Joined
Nov 10, 2013
Location
Lawrenceville PA
TDI
2003 TDI Jetta Daughters Car, 2001 TDI Beetle, Wife’s car, 2005 Golf TDI Mine, all 5 spds
If he was getting 45 with the 01M on a regular basis he was doing a lot of running along behind and pushing. 45 on a trip yes, not around town.

In fact his transmission may be failing. Does it go all the way through the gears and get up into fourth? If I recall correctly rpm's should be right around 2000 at 60mph.

Or as flee said, check the brakes especially the rears. The e-brake is a known dragging, fuel mileage robbing issue on these cars also.
 
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BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
Yes, maybe ask your buddy how many tanks the 45 was based on, and how many tanks the 35.
Many times folks will blur the facts when speaking of money, sex and MPG.
Sometimes posters even give us numbers based on the fuel gauge.
 

thechoochlyman

Vendor , w/Business number
Joined
May 7, 2015
Location
Campbellsville, Kentucky
TDI
1997 B4 Sedan
Thanks for the replies, guys.

He drives out of state rather frequently, and said he did always get 45 highway, 40 combined. He also mentioned something about a shudder he feels when letting off the fuel and coasting, so we're gonna do some brake diagnostics.

Sent from my HTC One_M8 using Tapatalk
 

UhOh

Top Post Dawg
Joined
Dec 24, 2014
Location
PNW
TDI
2000 & 2003 Golf GLS (2005 Mercedes E320 CDI)
Not sure why you altered the IQ. And, not sure why it was sitting at 5.3 (esp if it's stock). Only ever need to adjust the IQ if there's an issue. Adjusting it down would tend to make FE worse (though, I think, having it too high might not be good either): at any rate, unless someone has been messing around with the IQ there's little reason to suspect it having caused a recent change.

Need to log data. Either the car's intake has finally gotten to the point of it clogging enough to matter*, or something like the MAF is throwing things off. Or perhaps some of both.

* When I got my car it's performance was dismal. I was pretty certain, being new to TDIs and having researched this, that it was a clogged intake. The interesting thing to note here is that within only a matter of a few thousand miles the intake clogging increased enough to cause the speed/cruise control to start acting flaky: the car stopped holding speed when starting to go up slight inclines. After cleaning the intake (and EGR valve) the speed/cruise control worked (and has continued to do so for some 35k+ miles) perfectly.
 

thechoochlyman

Vendor , w/Business number
Joined
May 7, 2015
Location
Campbellsville, Kentucky
TDI
1997 B4 Sedan
I just went ahead and lowered the IQ because the car was entirely gutless... I knew it wouldn't help the mileage, but he also complained about how little power it had. Still doesn't have a ton, but it's better than before. I think he's also gonna try to get a Malone stage 2 flashed to his ECU eventually.

He also cleaned out his intake and egr valve not too long ago, and I reduced his egr cycle to the lowest setting. No idea how badly the head is clogged up though.

Sent from my HTC One_M8 using Tapatalk
 

UhOh

Top Post Dawg
Joined
Dec 24, 2014
Location
PNW
TDI
2000 & 2003 Golf GLS (2005 Mercedes E320 CDI)
How is FE being calculated/observed?

Messing with the EGR cycle isn't advisable because the ECU will adjust around your adjustment!

I'd set everything back to exactly the way it was. Focus on changing ONE thing at a time. Do logging and tell us what data you get.

BTW - How many RPMs are you seeing at 60 mph? Do they stay fairly stable/consistent?
 

thechoochlyman

Vendor , w/Business number
Joined
May 7, 2015
Location
Campbellsville, Kentucky
TDI
1997 B4 Sedan
Pen and paper with each fill up I would assume. I am not going to consider or suggest the possibility that his record keeping has somehow just gotten terrible all of a sudden.

I'll admit I'm just a 1Z guy, but how does reducing the EGR cycle in VCDS make the ECU compensate for it when I'm telling the ECU what to do? How would this in any way affect what else I'm doing?

I'll ask him to look at the RPMs - I'd assume you're wondering about the possibility of his transmission starting to go out? But I will try to do a log sometime soon.
 

UhOh

Top Post Dawg
Joined
Dec 24, 2014
Location
PNW
TDI
2000 & 2003 Golf GLS (2005 Mercedes E320 CDI)
People have reported FE based on realtime readouts or over a short distance. When I first installed tunes in my cars my Utragauges were telling me that the FE was crappy. Even with sub-par FE in these things it takes a while to empty out a tank (to then pull out pen and paper).

Do a search on adapting the EGR and you'll find more than you care to read about. Bottom line: don't do it.

Yeah, checking RPMs is an easy thing to do to evaluate whether the torque converter is locking up. I'd think, however, that someone that's fairly familiar with their car should recognize if there's a problem here. But, I'm not going to make any assumptions.
 
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